They are primarily Russian nationals working for a Russian commercial military provider that operates under Russian command in Syria. Some local mercenaries and paramilitary units may also answer to the Russians on an ad hoc basis. This method permits Putin to avoid concern in Russia over cost and Russian military casualties.
Well, if they want to die we will give them the opportunity.
We need to get out of these hellholes.
Solution: US get out of Syria. Use the money to safeguard our southern border - or give it back to hardworking Americans in a tax refund.